Int 13h buffer 64k boundaries

Guy Sotomayor Jr ggs at
Thu Apr 19 21:56:17 CDT 2018

> On Apr 19, 2018, at 4:16 PM, Chuck Guzis via cctalk <cctalk at> wrote:
> On 04/19/2018 12:14 PM, Fred Cisin via cctalk wrote:
>> I have no difficulty admitting that I didn't, and don't, have
>> Chuck's level of experience and knowledge. My entire venture into
>> microcomputers was a hobby that got out of hand.
> It's not so much expertise, but where you start your investigations.
> Right when I peered into the 5150, I saw the 8237 DMA controller (first
> cousin to the 8257) and recognized it from my 8-bit (8085) days.  It was
> immediately obvious that IBM had taken a bunch of legacy 8 bit
> peripheral chips and shoved them into the PC.   In fact, the 5150 was
> surprising in that how primitive the engineering was--something you
> didn't expect from a high-tech pioneer like IBM.  So the DMA address
> space had to be 16 bits with simple bank select--using a disk controller
> chip that was design to be used with 8 inch drives.

As I have mentioned previously, the 5150 was done by a relatively small
team and they leveraged hardware from a product that had been released
a short time prior to the 5150.  That product was the System/23 which was
based on the 8085.  The importance of the System/23 cannot be overstated
as it was the first IBM product that featured a non-IBM designed CPU.

It is also the case that the entire team that developed the 5150 HW and BIOS
were all from the System/23 team.  The XT-bus was the way it was because
it was the System/23 peripheral bus turned 180-degrees so that “cheap” PC
cards could not be used in the System/23.

The fact that it used “primitive” engineering was actually a design goal.  The
point of the 5150 was to create something that was simple to build and had
a simple design.  Due to the shoestring (for IBM) budget, the team leveraged
a lot from the System/23.

As to why IBM entered the PC market, the rumor was (at least at the time
within IBM) was that T.J. Watson, Jr. was at an employee’s house and saw
an Apple II.  He said that he wanted to have IBM branded computers in IBM
employees homes.  That was how the IBM PC project was kicked off.

BTW, I was on the System/23 team (wrote a fair amount of the ROM code)
and I knew all of the folks on the PC team.  Dr. Dave Bradley (of CTRL-ALT-DEL
fame) had the office across the hall from mine and discussed a lot of the
goings on for what would become the 5150.

TTFN - Guy

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